How can a senior person find out that s/he is taking adequate exercise for his/her age and not overdoing it?
Before you start any exercise, check with your doctor to find out adequate training for your age. But older adults must stay active as their abilities (mobility, stamina) and conditions allow since regular physical exercise is vital for healthy ageing.
Older adults need to do aerobic, muscle-strengthening, combined with stretching exercises weekly, and balance activities as required for substantial health benefits. Think long-term. As long as the build-up is gradual, allowing the body to adapt and exercise within the limits of each individual capability, there is no reason to prevent an older adult from avoiding any particular exercise. It does not matter whether it is high intensity or endurance.